1. Field of the Invention
The invention relates generally to high-energy radiation monitoring and detection; more particularly, it relates to an ion chamber for detecting high-energy radiation.
2. Background Art
Detectors of high-energy and ionizing radiations are used in various applications. Such detectors, for example, include ion chambers, proportional counters, Geiger-Mueller counters, and scintillation counters. Among these, ion chambers are commonly used in neutron detectors. FIG. 1 shows a basic system for neutron detection that includes a target chamber 13, an ion chamber 14, and electronics. Fast neutrons 12 are produced by a neutron source 11. These fast neutrons 12 interact with hydrogen nuclei in the target chamber 13 until their velocity is reduced to the average thermal velocity of the target. The thermal (slow) neutrons are then scattered from the target 13 to the ion chamber 14.
In a typical neutron detector, the ion camber 14 is filled with a gas (such as He-3) that can interact with the thermalized neutrons to produce ions. When an He-3 atom absorbs (captures) a thermalized neutron, a nuclear reaction occurs and the resultant products are a fast-moving tritium (H-3) atom and a proton. These fast-moving particles travel through the gas, pulling electrons in their wake and thus creating an equal number of positive and negative ions. When a potential is applied across the electrodes 15, 16 in the ion chamber 14, the ions are swept to the electrodes of opposite charges, producing currents that are directly proportional to the number of ions transferred. The number of ions transferred to the electrodes depend on the rates of their formation and hence the neutron flux. Thus, the ion currents measured by the ion chamber may be used to derive the magnitudes of the neutron flux.
However, the ion currents generated in these processes are extremely small (on the order of 10−12 amp), making it very difficult to accurately determine neutron flux. In addition, temperature and humidity changes in various electronic components, cables, etc. can further compromise the accuracy of the measurements. The situation is even worse under field conditions, which often include wide variations in temperature and humidity.
Furthermore, instability in leakage currents can also significantly degrade the accuracy of repeat measurements. Leakage current is a current through the detector system that is not due to ion transport through the ion chamber 14. Leakage currents can be due to cables, connections, parasitic current in the components, moisture contamination of the amplifier circuit or other components, or any number of other factors. Thus, leakage current depends on a highly convoluted function of temperature, humidity, age of components, and any number of other factors. Because the ion current in an ion chamber is on the order of 10−12 amp or less, leakage current can be a significant fraction of the total measured current, and any variation in the leakage current can significantly impact the accuracy of the measurements.
While the prior art ion chambers are capable of providing satisfactory measurements, there remains a need for ion chambers that can provide more reliable and accurate measurements of high-energy radiations.